The overall objectives of the project are to investigate the immune response of aged mice in terms of numbers of antibody forming cells and quality (avidity) of antibodies, and to identify the sites(s) of aged- related immunologic defects in terms of cell type and non-cellular environmental factors. An investigation of certain parameters of the immune response was begun in young (C57/BL x C3H) Fl mice. The results obtained will serve as a standard of reference with which the data obtained in aged mice will be compared. The variables examined, or being examined, include: number of plaque forming cells (PFC) as a function of time after secondary antigenic stimulation; secondary PFC response to varying doses of antigen; determination of minimal number of primed spleen cells capable of mounting a secondary response upon antigenic stimulation in vitro; determination of class of antibody produced by PFC during the course of the secondary response; changes in avidity of antibody produced by PFC as a function of length of immunization; frequency distribution of antibody avidity in the secondary response elicited by optimal and suboptimal antigen doses; B-cell and T-cell tolerance as a function of antigen dose.